1. Field of the Invention
The present invention relates to an autofocus apparatus for a camera and a method for controlling the same.
2. Description of the Related Art
Autofocus (AF) systems for moving a focus lens to a position where a maximum value of high-frequency components of a luminance signal output from an image sensor can be obtained as an in-focus position have been conventionally used for electronic still cameras and video cameras. An example of the AF system includes a hill-climbing system for moving a focus lens in a direction in which focus evaluation value based on high-frequency components of a luminance signal obtained from an image sensor (hereinafter referred to as a focus signal) increases and a position where the focus signal reaches its maximum value is determined to be an in-focus position.
Another example of the AF system includes a scanning system for moving a focus lens over the entire area within a movable range of a focus lens while storing a focus signal each time and determining a lens position where the maximum focus signal is obtained among the stored focus signals to be an in-focus position.
In this case, limiting the scanning range of a focus lens to a part thereof allows a time period to complete an auto-focus operation to be shorter than that when scanning the entire area thereof.
Japanese Patent Application Laid-Open No. 2006-293383 discusses a method for determining a target range when a part of a movable range of a focus lens is scanned. In this case, there is provided a photoelectric conversion unit different from an image sensor. The photoelectric conversion unit detects a distance between an imaging apparatus and an object to detect an in-focus position. A range in which the focus lens is moved is set based on the in-focus position.
Furthermore, the range is set narrower at a wide end side, while being set to widen toward a telephoto end according to the focal length of the lens.
On the other hand, a technique for repeatedly performing a focus adjusting operation for a moving object to keep an in focus-state has been known.
When the focus adjusting operation is repeatedly performed as described above, even if the amount of movement of the object is substantially constant, the amount of movement of the image forming position of an object image that has passed through an imaging lens differs depending on the distance to the object. The reason for this is the depth of field of the imaging lens.
Even if the amount of movement of the object is constant, for example, when the distance to the object is far, the depth of field is deep, so that the amount of movement of the image forming position is small. The reason for this is that when the distance to the object is near, the depth of field is narrow, so that the amount of movement of the image forming position is large.
Therefore, a time period required until when an object image is substantially defocused, that is, in an out-of-focus state, when the object moves out of the depth of field from the in-focus state, differs depending on the distance to the object.
When this is not taken into consideration to determine a time interval between a focus adjusting operation and a subsequent focus adjusting operation, i.e., a time interval in repeating the focus adjusting operation, to be constant regardless of the distance to the object, the following situations may arise.
That is, the focus adjusting operation is performed even when an object is at a far distance and within the depth of field. In contrast, the focus adjusting operation is not performed even when the object is at a near distance and out of the depth of field.
This results in situations that the object image is out-of-focus because the focus adjusting operation is unnecessarily performed, though the focus adjusting operation is not performed, while not performed when the focus adjusting operation must be performed.
Similarly when the movable range of the focus lens is set to be constant regardless of the distance to the object, a wider range may be scanned even though the amount of movement of focus position is small.
Conversely, a narrow range may be scanned even when the movable range is wide. As a result, a time period for focus adjustment is uselessly becomes longer. Alternatively, the object image cannot be in focus because the focus position is not included in the scanning range.
Similarly when the amount of movement of the focus lens is set to be constant regardless of the distance to the object, the amount of movement of the focus lens may be increased to scan a wider range even when the amount of movement of the focus position is small.
In contrast, the amount of movement of the focus lens may be decreased to scan a narrower range even when the movable range is wider. As a result, the time period for focus adjustment may become uselessly longer. Alternatively, the object image cannot be in focus because the focus position cannot be obtained.
Similarly when the focus adjusting operation is repeatedly performed, the time period for focus adjustment may become uselessly longer or the object image cannot be in-focus because the focus position cannot be obtained unless the time interval between the focus adjusting operation and the subsequent focus adjusting operation, the movable range in which the focus lens is moved, or the amount of movement of the focus lens is set in consideration of focus information indicating whether or not the object image is focused last time.